1. Field of Invention
The present invention relates to organic black matrices for color filter plate manufacture and to methods for making the same. It particularly relates to black matrices, having high optical density at ultra thin film thicknesses, excellent evenness, high strength, low cost and being useful for the production of a variety of display devices.
2. Background of the Prior Art
Multicolor liquid crystal displays (LCDs) are routinely produced having a thin, light-absorbent film, called a black matrix, applied to an array of color pixels which together form a color filter plate. The processing of such color filter plates remains one of the most troublesome steps during mass production of the LCDs because such processing normally employs a black matrix film made from sputtered chrome.
A spin-coatable, organic polymer based black matrix would tend to be more environmentally friendly than chrome, easier to reproduce, and offer lithographic processing advantages. However, there are at least two types of color filter plates for LCDs where organic polymer black matrices are woefully expensive and/or lack the desired performance. That is, the unavailability of an organic black matrix photoresist having sufficiently high optical density at ultra thin film thickness, has hampered the advancement of 1) thin film transistor (TFT)-arrays for TFT-LCDs and 2) super twisted nematic (STN) LCDs.
The optical density (O.D.) of the black matrix must be greater than 2.0 in order to block the transmission of light to the TFT or STN displays. Otherwise, photo leaks from non-display areas will reduce the contrast ratio and create adverse photo leakage current. In other words, one can enhance the contrast of the LCD by eliminating the light leakage which would otherwise occur through spaces patterned between the red, green, and blue (RGB) pixels on the color filter plate. The technical goal is to keep the light transmission at or below 1%, across the entire spectrum of from ultraviolet to infrared, at ultra thin black matrix film thicknesses.
It has proven extremely difficult, if not impossible, to manufacture an organic black matrix having an O.D. of .gtoreq.3.0 at ultra thin black matrix film thicknesses. Although O.D. greater than, or equal to, 2.0 have been achieved for organic black matrices at a 2 micron polymer thickness, such thick layers tend to cause a number of defects. For example, the so-called reverse tilt inside each pixel display area occurs at a gap of 2 microns. Reverse tilt causes defects in image and contrast deterioration. Overcoming that drawback, inter alia, requires an organic black matrix film having O.D. greater than 2.0 at a thickness of 1.0 micron or less.
Chrome Based Black Matrix
Therefore, despite
1) the high cost-of-ownership, PA1 2) the complexity of the sputtering process, PA1 3) the potential environmental problems, and PA1 4) a higher reflectance than desired,
the most common black matrix material has continued to be sputtered chrome. Vacuum evaporation and other coating techniques for metals such as nickel, aluminum and even chromium have been contemplated, but sputtering remains the most common technique and chrome remains the most common material, because other techniques and other materials, thus far, lack sufficiently high O.D. (.gtoreq.3.0) to provide enhanced contrast and high resolution, at sufficiently ultra thin thicknesses (1 micron or less) to be commercially effective for various display applications such as TFT and STN.
U.S. Pat. No. 5,378,274 by Yokoyama, et al., and U.S. Pat. No. 5,587,818 by S. Lee disclose typical chrome based black matrices for LCD's. That is, chromium is sputtered to form a thin film of about 2000 .ANG.. As previously discussed, such sputtering is an undesirably complex and expensive manufacturing process which poses serious environmental problems.
Dye Based Black Matrix Systems
U.S. Pat. No. 4,822,718 by Latham discloses an alternative potential black matrix for color filters. It contains a polyimide precursor vehicle and dyes. The color filters are used in color liquid crystal displays, light emitting diodes, photodiodes, and solid state lasers. However these black matrix compositions have several drawbacks, as a consequence of using dyes, rather than pigments, as the coloring agents. The dyed systems have lower heat, light and chemical resistance than desired for many applications. Moreover, dyes are not completely soluble in the solvent and polyimide vehicle system, thus the dyes fail to absorb light across a sufficiently broad spectrum of wavelengths. For example, Latham uses blue and brown dyes and, while not illustrating any data on optical density, we have that the desired optical density (.gtoreq.3.0) is not achievable using blue and brown dyes. Also, the dyed composition has long process steps which tend to lower production yield and raise fabrication cost.
U.S. Pat. No. 5,176,971 by Shimamura, et al., also discloses a dyed black matrix composition for color filters, which contains dyes and a polyimide precursor vehicle for use in color LCD devices. Its system is subject to fading as a consequence of the composition's limited heat stability. The dyes are said to have a relatively low thermal resistance. Additionally, a resin-surface modifier is added to the composition so as to impart water and oil repellency to the surface of the coating layer. The presence of excessive amounts of this surface modifier, however, impedes control of the uniformity of the thickness of the coating layer. Thus, even with the surface modifiers, the composition shows poor performance with respect to heat, light and chemical fastness. Furthermore, relative weight proportions of dye and polyimide must be carefully controlled; otherwise excessive dye will leak into the photoresist at a later step.
Pigment-Carbon Black Based Black Matrix Systems
It has been disclosed by Hesler, et al., in the article "Pigment-Dispersed Organic Black Matrix Photoresists for LCD Color Filters," SID Digest, 26:446 (1995); that carbon black dispersed in acrylic polymer provided an average optical density of 2.8 for 1.5 .mu.m film thickness. However it reports poor reproducibility and poor coating properties. The composition needed improvement in image quality.
As disclosed by Yamanaka, et al., in his article, "Integrated Black Matrix on TFT Arrays," SID Digest, 23:789 (1992); carbon black, even with advanced acrylic photo polymer, does not achieve OD greater than 2.0, at thickness less than 2 .mu.m without creating crosstalk. Yamanaka also describes the disadvantage of a 2 .mu.m "step size" (or thickness). It is so large that it results in reverse tilt inside each pixel display area. This causes spot defects in image and contrast deterioration.
Hasumi, et al., in his article "Carbon Dispersed Organic Black Matrix on Thin Film Transistor," Proc. of Int. Display Res. Conf. (EuroDisplay-96), 16:237 (1996) also discloses a carbon black dispersed system providing OD more than 2.0 at 1.5 .mu.m film thickness, but presumably, like Yamanaka, it is also subject to crosstalk.
U.S. Pat. No. 5,368,976 by Tajima, et al., discloses another pigment-dispersed color filter composition useful for the production of LCD and charged coupled devices. The system requires controlling the molecular weight (M.W.) of a radiation-sensitive acrylic block copolymer vehicle. If M.W. exceeds 500,000, then scum develops around the pixels, causing insufficient sharpness at pattern edge, surface soiling, and excess resin at non-pixel portions. If M.W. is less than 10,000, the development time margin is deleteriously decreased. The radiation-sensitive copolymer composition promotes adhesion to the substrate, facilitates transmittance and aids in color contrast. The acrylic type copolymer has limited thermal stability and poor photospeed. The pigment, copolymer and a radiation sensitive component are dispersed via grinding in a ball mill for 18 hours which is unduly time consuming. The dispersed pigment is filtered through a 10 .mu.m filter which allows pigment particle sizes in the formulation which fabricate into low transparency film. The shelf life of one week in a dark place at room temperature is too short. The dispersion needs dark conditions and is not stable after one week. The composition uses Pigment Black 1 and Pigment Black 7 only. Pigment Black 1 does not provide high optical density and the process does not use any dyes.
In summary, the previous black matrices have several drawbacks which may be listed as follows:
Chrome based black matrix suffers from, inter alia, high cost, poor adhesion, and toxicity concerns.
Dyed matrices are limited by the generally lower absorptivity of black dyes in comparison to black pigments and the limited solubility of such dyes either in solvents or a polymer matrix. Also, dyed formulations do not provide sufficient heat, light and chemical stability.
Pigmented coatings suffer from one or more of the following problems: 1) the formulations are unstable against gelation or flocculation at the required high carbon black loadings, making it very difficult to have long shelf life for the pigmented systems; 2) if the formulations are designed to be photosensitive, they can not be too highly loaded with carbon black without losing their imaging properties; and 3) formulations containing non-carbon black pigments do not provide very high optical density, while 4) carbon black pigments have particle sizes too large to permit sufficiently thin layer thickness at the requisite high optical densities.
The cited prior art reveals the need for a black matrix composition with very high optical density (.gtoreq.3.0), and with heat, light, and chemical fastness and longer shelf life. The prior art also illustrates the need for a composition with low cost, less manufacturing (and processing) time, long shelf life, and environmentally safe.